Recently, a plasma display panel (hereinafter referred to as “PDP”) used for image display in a computer or a television has been demanded to be not only enlarged, thinned, and lightened in weight, but also increased in definition to achieve high image quality.
In order to display a high-quality image by controlling the panel so that light emission is secured in a discharge cell to emit light and no light emission is secured in a discharge cell to emit no light, a certain address operation is required within an assigned time. For this purpose, a panel capable of being driven at a high speed has been developed, and a driving method and driving circuit for exploiting the performance of the panel and displaying a high-quality image have been studied.
FIG. 24 is an example of a waveform chart of conventional driving voltage applied to each electrode of the PDP. FIG. 24 shows a conventional example of the driving voltage waveform in subfields. FIG. 24 shows a driving waveform of scan electrodes (SCN1-n in FIG. 24), a driving waveform of sustain electrodes (SUS1-n in FIG. 24), and a driving waveform of address electrodes (D1-m in FIG. 24). Before address period 32 when address discharge for selecting a lit cell is performed, wall charge desired for address discharge is accumulated by weak discharge in the initializing period. The first subfield (hereinafter referred to as “SF”) in one field has all-cell initializing period 31 when all-cell initializing operation for causing initializing discharge in all the discharge cells to display an image is performed. The other subfields have selective initializing period 34 when selective initializing operation for causing initializing discharge only in the cell having undergone all-cell initializing operation or sustain discharge in the preceding subfield is performed.
In address period 32, a cell to be lit by address discharge is selected. In sustain period 33, sustain operation of sustaining light emission only in the cell having undergone the address discharge in address period 32 is performed. In the initializing operation in the first half of all-cell initializing period 31 in the first SF, all sustain electrodes SUS1 through SUSn and all address electrodes D1 through Dm are kept at 0 V. Ramp voltage gradually increasing to voltage Vh, which is threshold voltage Vff or higher, is applied to all scan electrodes SCN1 through SCNn, and gas discharge occurs in a discharge section of the PDP. Here, at threshold voltage Vff, the discharge starts between scan electrodes SCN1 through SCNn and sustain electrodes SUS1 through SUSn and between scan electrodes SCN1 through SCNn and address electrodes D1 through Dm. Sustain electrodes SUS1 through SUSn are paired with scan electrodes SCN1 through SCNn, and address electrodes D1 through Dm intersect with them. The discharge is weak discharge where ionization multiplication increases temporally gradually. The charge generated by the weak discharge is accumulated as wall charge on a wall surface surrounding the discharge section so as to reduce the electric field inside and on the surface of the discharge section around data electrodes, the scan electrodes, and the sustain electrodes. Negative charge is accumulated as wall charge on the surface of a protective film near the scan electrodes, and positive charge is accumulated as wall charge on the surface of a protective film near the sustain electrodes and on the surface of a phosphor layer near the address electrodes. In the initializing operation in the latter half of all-cell initializing period 31, all sustain electrodes SUS1 through SUSn are kept at positive voltage Ve. Ramp voltage gradually decreasing to voltage Vbt, which is threshold voltage Vpf or lower, is applied to all scan electrodes SCN1 through SCNn, and gas discharge occurs in a discharge section. Here, at threshold voltage Vpf, the discharge starts between scan electrodes SCN1 through SCNn and sustain electrodes SUS1 through SUSn and between scan electrodes SCN1 through SCNn and address electrodes D1 through Dm. Sustain electrodes SUS1 through SUSn are paired with scan electrodes SCN1 through SCNn, and address electrodes D1 through Dm intersect with them. The discharge is also weak discharge where ionization multiplication increases temporally gradually. This weak discharge reduces the negative charge accumulated on the surface of the protective film near the scan electrodes, and the positive wall charge accumulated on the surface of the protective film near the sustain electrodes.
In a state where all electrodes are grounded after the all-cell initializing operation, a desired potential difference (hereinafter referred to as “wall potential”) required for selecting a lit cell with address discharge is caused by the accumulated wall charge between the scan electrodes and the address electrodes and between the scan electrodes and the sustain electrodes. The initializing operation means operation of producing, with discharge, a desired wall charge for controlling the address discharge. In address period 32, voltage lower than that of the data electrodes and the sustain electrodes is applied to the scan electrodes. Voltage is applied only to the address electrode of the cell to be lit so as to cause the voltage difference of the same sign as that of the wall potential between the scan electrode and the address electrode. Thus, address discharge occurs. Negative charge is accumulated as wall charge on the surface of the phosphor layer and on the surface of the protective film near the sustain electrodes, and positive charge is accumulated as wall charge on the surface of the protective film near the scan electrodes. In a state where all electrodes are grounded after the address operation, a desired wall potential required for causing sustain discharge between the scan electrodes and the sustain electrodes is generated by wall charge.
In sustain period 33, firstly, voltage higher than that of the sustain electrodes is applied to the scan electrodes to cause discharge. Then, voltage is applied so that the polarity of the scan electrodes and the polarity of the sustain electrodes interchange, thereby intermittently keeping light emission. In selective initializing period 34, rectangular waveform erasing voltage where the phase difference time width from the scan electrodes is narrow is applied to the sustain electrodes, thereby causing the incomplete discharge to extinguish a part of the wall charge and preparing for the initializing operation of the next SF. In the driving method of the conventional PDP, an image is displayed by a sequence of the initializing period, the address period, and the sustain period.
The discharge characteristic of the panel largely depends on the characteristic of the protective layer. Especially, in order to improve the electron emission performance and charge retention performance affecting the possibility of high-speed driving, the material, structure, and manufacturing method of the protective layer have been studied widely. Patent literature 1, for example, discloses a panel having a magnesium oxide layer that is produced by gas phase oxidation of magnesium vapor and has a cathode luminescence emission peak at a wavelength of 200 to 300 nm. Patent literature 1 also discloses an electrode driving circuit for sequentially applying scan pulses to one electrode of each of the display electrode pairs forming all display lines in the address period and for applying, to the data electrode, the address pulse corresponding to the display line to be applied with the scan pulse.
In the conventional PDP (conventional example 1), in all-cell initializing period 31 for accumulating a desired wall charge with weak discharge, the number of charged particles causing discharge reduces absolutely in the following cases:                the density of ions and electrons (charged particles causing ionization multiplication) initially existing in the discharge section is low; or        phosphors and barrier ribs apt to absorb the charge of the charged particles surround the discharge section.Therefore, occurrence probability of strong discharge (hereinafter referred to as “strong discharge”) where ionization multiplication increases temporally sharply becomes high. When the strong discharge occurs, more excessive wall charge (which substantially cancels the electric field in the discharge section) than the desired wall charge is accumulated, and abnormal wall potential higher than a desired wall potential occurs. Disadvantageously, the action of the abnormal wall potential causes sustain light emission though the cell is unlit in the sustain period, and normal image display is not allowed.        
When video display is performed using a high definition PDP, there are the following problems. For example, in a progressive 42-type full high-definition (HD) PDP (1920×1080 pixels) increased in definition, the cell pitch is short and hence the influences of electric field interference with an adjacent cell and scattering of charged particles are increased even when cells are separated from each other by the barrier ribs. In the conventional PDP driving method (conventional example 2) shown in FIG. 24, rectangular waveform voltage is applied in selective initializing period 34, so that erasing discharge becomes strong. Therefore, when the high definition PDP is driven in conventional example 2, the influence of discharge interference between adjacent cells in the initializing period becomes large, a desired wall potential cannot be accumulated in the address operation, and the address operation cannot be performed normally, disadvantageously (for example, patent literature 2).
In the conventional PDP, the electron supply amount for performing stable initializing operation is short in the following cases:                the pixel pitch is small due to high definition and the ratio of the surface area to the volume of the discharge section is high; and        the mixing ratio of discharge gas such as xenon or krypton having a large atomic number is increased in order to increase the luminance.Then, strong discharge occurs in the initializing period, the abnormal wall charge accumulated by the strong discharge causes sustain light emission though the cell is unlit in the sustain period, and hence normal image display is not allowed. This is a first problem.        
In the conventional driving method, when the high definition PDP is driven, the influences of electric field interference between adjacent cells and scattering of charged particles in the selective initializing period are large, sustain light emission does not occur though the cell is lit in the sustain period, and normal image display is not allowed. This is a second problem.
The reason why improving the definition makes the first problem larger is described in detail. Following the improvement in definition, the volume of the discharge section per cell decreases, the ratio of the surface area to the volume of the discharge section increases, the energy loss due to re-absorption of the charged particles on the wall surface and due to heating caused by elastic collision increases, and more electric power is required to be supplied from the outside. As a result, the number of charged particles inside the discharge section before the all-cell initializing operation decreases, and the driving voltage increases in each period. When the voltage applied to the electrodes increases, the electric field intensity inside and on the discharge section around the electrodes increases, and the probability of temporally sharply increasing the ionization multiplication becomes higher. As a result, it becomes more difficult to cause the weak discharge used in the conventional initializing operation.
Thus, the number of charged particles inside the discharge section is decreased and the driving voltage is increased by improvement in definition, so that the strong discharge is apt to occur in the initializing period. As a result, it becomes more difficult than the conventional art to normally select a lit cell or an unlit cell in the address period.
The improvement in definition reduces the size of one cell, so that the light shielding factor by the barrier ribs and metal electrodes increases, the luminance reduces, and the whole video becomes dark. As a method of securing the luminance required for high quality image display, a method of increasing the whole pressure of the discharge gas or the mixing ratio of xenon or krypton contributing to the emission of visible light receives attention. For example, it has been studied that the whole pressure is between 180 and 750 Torr inclusive and the xenon partial pressure ratio is 10%, 15%, 20%, 30%, 50%, 80%, 90%, 95%, 98%, or 100%.
The reason why the higher mixing ratio of xenon or krypton makes the first problem larger is described in detail. An element such as xenon or krypton having a large atomic number has small electronic energy (first ionization energy) on its outermost shell, so that secondary electron emission coefficient is extremely smaller than that of helium, neon, argon having large electronic energy on its outermost shell. As a result, the absolute number of electrons supplied from the surface of the protective layer to the discharge section decreases, and the threshold voltage required for starting discharge increases. When the voltage applied to the electrodes increases, the electric field intensity inside and on the discharge section around the electrodes increases, and the probability of temporally sharply increasing the ionization multiplication becomes higher. As a result, it becomes more difficult to cause weak discharge used in the conventional initializing operation.
Also when the partial pressure ratio of xenon or krypton is increased to secure high luminance required for high quality image display, strong discharge is apt to occur in the all-cell initializing period. When strong discharge occurs, the contrast ratio extremely decreases because the light emission intensity by one discharge is strong, and the image quality extremely degrades in video having many low gradation expressions. As a result, it becomes more difficult than the conventional art to normally select a lit cell or an unlit cell in the address period.
[Patent Literature 1] Unexamined Japanese Patent Publication No. 2006-54158
[Patent Literature 2] Unexamined Japanese Patent Publication No. 2000-214823
[Patent Literature 3] Unexamined Japanese Patent Publication No. 2007-48733